Medium fiend (yugoloth), neutral evil
Large fiend (yugoloth), neutral evil (suggested)

Armor Class 15
Armor Class (suggested) 18
Hit Points 135 (18d8 + 54)
Hit Points (suggested) 153 (18d10 + 54)
Speed 20 ft., swim 40 ft.

Proficiency Bonus +4
Proficiency Bonus +7 (5th Edition Advanced Mode)

STR	DEX	CON	INT	WIS	CHA
12 (+1)	21 (+5)	16 (+3)	19 (+4)	10 (+0)	14 (+2)

Saving Throws (suggested) Dex +9, Con +7, Int +8
Skills Insight +4, Perception +4
Damage vulnerabilities fire
Damage Resistances cold, lightning; bludgeoning, piercing, and slashing from nonmagical attacks
Damage Immunities acid, poison
Condition Immunities poisoned
Senses blindsight 60 ft., darkvision 60 ft., passive Perception 14
Languages Abyssal, Infernal, telepathy 60 ft.
Challenge 9 (5000 XP)

Amphibious. The hydroloth can breathe air and water.

Innate Spellcasting. The hydroloth’s innate spellcasting ability is Intelligence (spell save DC 16). It can innately cast the following spells, requiring no material components:

• At will: darkness, detect magic, dispel magic, invisibility (self only), water walk
• 3/day each: control water, crown of madness, fear, phantasmal killer, suggestion

Magic Resistance. The hydroloth has advantage on saving throws against spells and other magical effects.

Magic Weapons. The hydroloth’s weapon attacks are magical.

Secure Memory. The hydroloth is immune to the water of the River Styx as well as any effect that would steal or modify its memories or detect or read its thoughts.

Watery Advantage. While submerged in liquid, the hydroloth has advantage on attack rolls.

ACTIONS

• Multiattack. The hydroloth makes two melee attacks. In place of one of these attacks, it can cast one spell that takes 1 action to cast.
• Multiattack (suggested). The hydroloth makes three melee attacks: one with its bite and two with its claws. In place of one of these attacks, it can cast one spell that takes 1 action to cast.
• Claws. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 14 (2d8 + 5) slashing damage.
• Claws (suggested). Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 9 (1d8 + 5) slashing damage.
• Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 16 (2d10 + 5) piercing damage.
• Poison Spit (suggested, recharge 5-6). The hydroloth can use its bonus action to spit a stream of foul yellow liquid at one creature within 10 feet of it. The target must make a DC 15 Constitution saving throw, taking 8 (1d10 + 3) poison damage on a failed save, or half as much damage on a successful one. Additionally, on a failed save, the target falls unconscious for 1 minute. The target wakes up if it takes damage or if another creature uses an action to shake it awake.
• Steal Memory (1/Day). The hydroloth targets one creature it can see within 60 feet of it. The target takes 4d6 psychic damage, and it must make a DC 16 Intelligence saving throw. On a successful save, the target becomes immune to this hydroloth’s Steal Memory for 24 hours. On a failed save, the target loses all proficiencies, it can’t cast spells, it can’t understand language, and if its Intelligence and Charisma scores are higher than 5, they drop to 5. Each time the target finishes a long rest, it can repeat the saving throw, ending the effect on itself on a success. A greater restoration or remove curse spell cast on the target ends this effect early.
• Teleport. The hydroloth magically teleports, a long with any equipment it is wea ring or carrying, up to 60 feet to an unoccupied space it can see.

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Hydroloth

A hydroloth is a type of fiendish creature known as a yugoloth, which are mercenaries and manipulators that dwell in the lower planes. Hydroloths are amphibious and have a reptilian appearance, with scales, claws, and a long tail. They have yellow eyes and a gland under their tongue that can spit a stream of poisonous liquid.

Description

Hydroloths are cunning and ruthless, always looking for opportunities to gain an advantage over their enemies or allies. They are loyal only to themselves and their current employers, and will betray anyone if it suits their interests. They have a special ability to steal the memories of creatures they attack, which they can use to blackmail, impersonate, or deceive others. They can also cast spells innately, using their intelligence as their spellcasting ability.

Hydroloths are immune to the effects of the River Styx, a river that flows through the lower planes and erases the memories of those who touch it. They can also communicate telepathically with any creature within 60 feet of them, and resist most types of damage except fire.

Hydroloths are squat humanoids with underarm gliding flaps and frog-like faces. They have warty yellow skin that secretes a slimy substance that helps them swim faster and escape from grapples. They can breathe both air and water, and can survive in fire or lava as well.

Combat

Hydroloths prefer to ambush their foes from underwater or use their teleportation ability to surprise them. They can make two melee attacks with their claws or bite, or use one of their attacks to cast a spell. Their most dangerous weapon is their poison spit, which can knock out a creature for several rounds or even kill them if they fail to resist its effects. Hydroloths have advantage on attack rolls while submerged in liquid.

Hydroloths are not very brave, and will flee or surrender if they are outnumbered or outmatched. They will also try to bargain for their lives or offer information in exchange for mercy. However, they are also very treacherous, and will not hesitate to break their promises or stab their captors in the back if they get a chance.

Hydroloths are elite fighting forces commonly used in yugoloth ambushes or amphibian attacks. They travel in groups of 10 or more wherever Styx enters the lower planes. They delight in fighting the Blood War and draw strength from the terror they inflict on mortals.

Habitat / Society

Hydroloths inhabit the waters of the River Styx, which connects the various layers of the lower planes. They often serve as guides, smugglers, or spies for other fiends who wish to travel across the river or avoid its dangers. They also work as mercenaries or assassins for anyone who can pay them well enough, whether it is a demon lord, a devil prince, or a mortal warlock.

Hydroloths have no loyalty or allegiance to any faction or cause, and will switch sides whenever it is convenient for them. They are also very secretive and paranoid, hiding their true identities and motives from everyone. They rarely cooperate with other yugoloths or hydroloths, unless they have a common enemy or a lucrative contract.

Hydroloths use their prestige among their betters to gain power and influence: extra wealth, or a share of the mortal life forces paid for the job they do. Greater yugoloths have the strength to force the hydroloths’ obedience, but they wisely appease these creatures’ demands.

Ecology

Hydroloths are unnatural creatures that were created by ancient evil beings known as baernoloths, who experimented with different forms of life and magic in their quest for power. Hydroloths are among the oldest and most primitive types of yugoloths, and have not changed much since their creation.

Hydroloths do not reproduce naturally, but can be summoned by powerful magic or rituals. They do not need to eat or drink, but they enjoy devouring the flesh and memories of their victims.

Hydroloths have few natural predators, as most creatures avoid the River Styx and its dangers. However, they sometimes clash with other aquatic fiends such as aboleths, krakens, or marids, who view them as intruders or rivals in their domains.

Several internal organs of hydroloths make excellent spell components and alchemical mixtures for potions. A live hydroloth, though extremely difficult to capture, might bring 2,500 gp on some parts of the Prime Material Plane or even (perhaps) in Sigil. A dead one, though not as valuable, still fetches upwards of 1,000 gp.